1. Field of the Invention
The present invention relates to detection of a defocus during recording in an optical disk unit for recording/reproducing information to/from a recording type optical disk having a plurality of recording layers.
2. Description of the Related Art
In recent years, the recording type optical disk having a plurality of recording layers has been put into practice. In the event that a defocus attributable to any external disturbance takes place in the course of recording data onto the recording type optical disk, there is a possibility that an in-focus point of a laser beam used for recording will pass through another recording layer. If the in-focus point of the laser beam passes through the different layer while the laser beam being conditioned to emanate at recording power, an erroneous record is caused in the layer the beam transmits through. Accordingly, when a defocus is detected during recording, the optical disk unit quickly reduces emission power of laser to a sufficiently low level to prevent an erroneous record on the different recording layer.
For detection of a defocus, JP-A-2001-67686 (Patent Document 1), for instance, discloses a method according to which a signal indicative of the sum of quantities of light reflecting from an optical disk (hereinafter referred to as a SUM signal) is compared with a predetermined level.
Also, Japanese Patent No. 3456040 (Patent Document 2), for instance, discloses another defocus detection method in which a focus error signal (hereinafter referred to as a FE signal) is compared with a predetermined threshold value